Iron ore concentrate pellets



1 0 6 8 4 4 CROSS REFERENCE ExAMxNEq M y 13, 1952 F. D. DE VANEY 2,596,132

IRON ORE CONCENTRATE PELLETS Filed July 31, 1947 Is an 6 lbs. of Addition Agent long Ton of conccniran E ffac/ af Add/flan A stre rig thlbsl ellet N 3 FRED D. DEVd/VEY helmwazdl 1 a Compressive Patented May 13, 1952 IRON ORE CONCENTRATE PELLETS Fred D. De Vaney, Hibbing, Minn., assignor to Erie Mining Company, Hibbing, Minn., a corporation of Minnesota Application July 31, 1947, Serial No. 765,167

nus invention relates to the art of forming pellets or small balls of finely divided solids, e. g., finely divided ore material. While not restricted thereto, the invention is concerned with an improvement in forming pellets of :goncentrates,

3 Claims. (Cl. 75-3) or other finely divided particulate fFrFs, of

oxidic iron, and will be described with particular 'fife'rence to the latter.

In the beneflciation of iron ore materials (e. g., low grade iron ores, washing plant tailin s ferruginous mown as taconite, and the e w ere y o obtain a concentrate desirably rich in oxidic iron and poor in gangue materials, it is necessary to subdivide more or less the starting material in order to practice a beneficiation process on the latter, with the result that the beneficiated product, or concentrate," is in a particulate, more or less e y v ded, form. Before such concentrates may be charged into the blast furnace they must be nodulizgg or sintered or briquetted or pelletized or o erwise formed into coherent aggregates or masses of substantial size, and the aggregates or masses must have a substantial degree of mechanical strength, e. g., a degree of mechanical strength comparable to that of merchantable ore."

The present invention is concerned with pelletizing as the mode of aggregation. In that process, the initially moist particles of oxidic iron are rolled into ball-like masses or pellets by the procedure described in my application Serial No. 675,675, filed June 10, 1946, now Patent 2,543,898, entitled Pelletizing Ore Fines-(see also Firth Patent No. 2,411,973)--which in the freshly prepared or fraw state have a remarkable mechanical strength but when dried are very feeble (as concentrate containing the optimum water content for making the most rugged pellets therefrom, where the compressive strength of the freshly formed (moist) pellet was taken as unity, the compressive strength of the pellet when dehydrated at about 220 F. was only a small fraction of unity and hence the dry pellet tended to fall into dust, whereas the dehydrated pellet when further heated to about 400 F. reacquired a compressive strength of unity and acquired progressively enhanced compressive strengths by being heated to progressively higher temperatures, e. g., seven times its original compressive strength when heated to ISO-800 F.

I have discovered that in the carrying out of the initial stages of thermal hardening treatment the pellets may be carried through the aforesaid intermediate or transitory period of greatest fragility, with materially lowered amount of disintegration of pellets, by adding an exceedingly small amount, of the order of a few pounds per ton (dry weight) of concentrate, of a suitable addition material (hereinafter to be described more-particularly) to, and homogeneously dispersing the same through, a mass or body of moist particles prior to the formation of the latter into pellets. The amount of addition material so incorporated into the mass is wholly insufficient to exert any significant eiIect, one way or the other, on the mechanical strength of the fully indurated pellets but does exert a surprising effect during the aforesaid intermediate stage when the pellets have become dry (or practically so) but have not started to become indurated.

Operable addition materials are adhesives, particularly wand sodium silicaltg The influence of starch 1s particu ar y s r ng, it being a fact that the incorporation of as little as 1 pound of starch into awater-wet mass of concentrate containing 1 long ton (dry weight) of oxidic iro particles confers upon the resulting pellets, after drying, nearly three times the compressive strength of a comparable dry pellet containing no addition material, and that the mechanical strength increases mathematically with increased addition of starch to 5 lbs/long ton. It is. a curious fact that increasing the addition of starch in excess of 5 lbs/long ton gives only a slight further improvement in the mechanical strength of the dried pellet. The influence of bIlUCiD Kill LKLliUlZ sodium silicate, while less striking than that of starch, is positive.

The graphs of the single figure of the appended drawing depict the relative influences of additions of varying amounts of starch and sodium silicate, in pounds per long ton of iron oxide particles, on the compressive strengths in pounds per pellet of the dried but not indurated pellets, the compressive strength of a dried pellet of iron oxide concentrate containing no addition material being about 0.8 pound per pellet.

It is altogether surprising, and could not have been foreseen from a knowledge of the prior art of briquetting and pelletizing, that so tiny amounts of starch (or sodium silicate) should have this very pronounced effect in strengthening pellets of iron ore concentrates during their period 01 greatest fragility in the induration process. The phenomenon probably is explainable only on this basis: in the protracted rolling-up operation, during which the growing pellets are compacted under substantial pressures (e. g., equivalent to 15,000 pounds per square inch), some or a substantial proportion of the total moisture content of the material (that amount necessary for securing plasticity and varying between 6 and 30%, by weight, depending on the particle size and particle size distribution of the material being pelletiaed) is worked out toward or into the surface layers of the pellet, the starch (or sodium silicate) migrates somewhat in the same direction as the moisture whereby to give a relatively low concentration of the addition material in the core of the pellet,

a relatively high concentration of the addition material in the surface layers of the pellet and a progressively increasing addition material content radially outwardly from the core to the surface layers. Examination of dried (212 F.) pellets in which starch or sodium silicate had been incorporated establishes that the outside portion is much harder (mechanically stronger) tha the center portion.

.Specific example The starting material was a filter cake product, containing about 10.8% by weight of free water, of oxidic iron conoentrate which was essentially'a minus s and at least 50% was minus 325 mesh. The filter cake product was fed to a conventional pug mill together with an amount of starch equal to pounds per long ton dry weight) of tee concentrate, and the starch was homogeneously dispersed through the filter cake product by thorough pugging. The thoroughly pugged material was fed by a belt conveyor into the upper end of a balling-up drum and therein was formed into pellets in the manner and by the technique described in Firth Patent No. 2,411,873, except that at the conclusion of the rolling-up operation and as the fully formed pellets were about to discharge from the drum they were dusted with a relatively very small amount, e. g.. 5% by weight, more or less, of dry oxidic iron ore fines as described and claimed in my application Serial .No. 675,675 before mentinned.

The resulting pellets were dried in an oven at 220 F. to constant weight, and were subjected to compressive strength tests. The dry pellets were found to withstand a compression of 6.4

"pounds per pellet average.

In a comparison experiment in which all of the above conditions were observed except that starch was omitted, the dry pellets had a compressive strength of 0.8 pound per pellet, average.

The above experiment was repeated using 1 pound/long ton, 2 pounds/long ton and 10 pounds/long ton, respectively, of starch. After the resulting pellets were dried to constant weight their compressive strengths were found to be as follows:

Starch addition lbs/pellet 1 lb./long ton (045%) About 2 2 lbs/long ton (089%) 3.15 10 lbs/long ton (45%) 7.67

In repetitions of the above experiment in which varying amounts of sodiumsilicate (water glass) were substituted for the starc e res mg p lets after being dried to constant Weight were found to have the following compressive strengths:

Sodium silicate addition lbs/pellet .1 lb./long ton (045%) 1.75 2 lbs/long ton (089%) 2.3 5 lbs/long ton (22%) 2.85 10 lbs/long ton (.4593) 3.4

Freshly prepared rolled pellets of finely divided oxidic iron concentrate made in accordance with the present invention contain from about 6 to about 30% moisture by weight and from about 0.015 to about 0.45% by weight of the added adhesive material.

I claim:

1. Process of preparing indurated pellets of finely divided oxidic iron concentrates which comprises forming a plastic mixture consisting essentially of the concentrates, water and starch, by mixing the finely divided oxidic iron concentrates and from 6 to 30% by weight of water and homogeneously dispersing starch through the mixture in the proportion of from 1 to 5 pounds per long ton. dry weight, of the finely divided oxidic iron concentrates, forming the resulting mixture into small masses, protractedly rolling the small masses into raw pellets in an inclined rotary drum thereby effecting migration of starch within the pellets towards the surface layers of the pellets causing the concentration of the starch content to be hi hest in the outside portions of the pellets and to diminish toward the center portions, and subjecting an assemblage of the raw pellets to thermal hardening treatment during which the raw pellets are dried and thereupon are heated to a temperature materially in excess of 400 F. but below the fusion temperature of the oxidic iron.

2. Process of preparing indurated pellets of finely divided oxidic iron concentrates which comprises forming a plastic mixture consisting essentially of the concentrates, water and starch, by mixing the finely divided oxidic iron concentrates and from 6 to 30% by weight of water and homogeneously dispersing through the mixture 9. small but effective amount of starch not exceeding 5 pounds per long ton, dry weight, of the finely divided oxidic iron concentrates, forming the resulting mixture into small masses, protractedly rolling the small masses into raw pellets in an inclined rotary drum thereby effecting migration of starch within the pellets towards the surface layers of the pellets causing the concentration of the starch content to be highest in the outside portions of the pellets and to diminish toward the center portions, and subjecting an assemblage of 5 the raw pellets to thermal hardening treatment during which the raw pellets are dried and thereupon are heated to a temperature materially in excess of 400 F. but below the fusion temperature of the oxidic iron.

3. Process of preparing indurated pellets of finely divided ore material which comprises forming a plastic mixture consisting essentially of the finely divided ore material, water and starch, by mixing the finely divided ore material and from 6 to 30% by weight of water and homogeneously dispersing through the mixture a small but efiective amount of starch not exceeding 5 pounds per long ton, dry weight, of the finely divided ore material, forming the resulting mixture into small masses, protractedly rolling the small masses into raw pellets in an inclined rotary drum thereby effecting migration of starch within the pellets towards the surface layers of the pellets causing the concentration of the starch content to be highest in the outside portions of the pellets and to diminish toward the center portions, and subjecting an assemblage of the raw pellets to 6 thermal hardening treatment during which the raw pellets are dried and thereupon are heated to a temperature materially in excess of 400 F. but below the fusion temperature of the ore material FRED D. DE VANEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Proceedings Blast Furnace and Raw Materials Committee, vol. 4, 1944, pages 54-60. 

1. PROCESS OF PREPARING INDURATED PELLETS OF FINELY DIVIDED OXIDIC IRON CONCENTRATES WHICH COMPRISES FORMING A PLASTIC MIXTURE CONSISTING ESSENTIALLY OF THE CONCENTRATES, WATER AND STARCH, BY MIXING THE FINELY DIVIDED OXIDIC IRON CONCENTRATES AND FROM 6 TO 30% BY WEIGHT OF WATER AND HOMOGENEOUSLY DISPERSING STARCH THROUGH THE MIXTURE IN THE PROPORTION OF FROM 1 TO 5 POUNDS PER LONG TON, DRY WEIGHT, OF THE FINELY DIVIDED OXIDIC IRON CONCENTRATES, FORMING THE RESULTING MIXTURE INTO SMALL MASSES, PROTRACTEDLY ROLLING THE SMALL MASSES INTO RAW PELLETS IN AN INCLINED ROTARY DRUM THEREBY EFFECTING MIGRATION OF STARCH WITHIN THE PELLETS TOWARDS THE SURFACE LAYERS OF THE PELLETS CAUSING THE CONCENTRATION OF THE STARCH CONTENT TO BE HIGHEST IN THE OUTSIDE PORTIONS OF THE PELLETS AND TO DIMINISH TOWARDS THE CENTER PORTIONS, AND SUBJECTING AN ASSEMBLAGE OF THE RAW PELLETS TO THERMAL HARDENING TREATMENT DURING WHICH THE RAW PELLETS ARE DRIED AND THEREUPON ARE HEATED TO A TEMPERATURE MATERIALLY IN EXCESS OF 400* F. BUT BELOW THE FUSION TEMPERATURE OF THE OXIDIC IRON. 